CELESTIAL SPECTROSCOPY. 73 
the energy set free in these changes, very different from those corre- 
sponding to the mean temperature of the tlame. 
Under the ordinary conditions of terrestrial experiments, therefore, 
the temperature or the mean vis viva of the molecules may have no di- 
rect relation to the total radiation, which, on the other hand, is the 
sum of the radiation due to each Iuminous molecule. 
These phenomena have recently been discussed by Ebert from the 
standpoint of the electro-magnetic theory of light. 
Very great caution is therefore called for when we attempt to reason 
by the aid of laboratory experiments tothe temperature of the heavenly 
bodies from their radiation, especially on the reasonable assumption 
that in them the luminosity isnot ordinarily associated with chemical 
changes or with electrical discharges; but is due to a simple glowing 
from the ultimate conversion of the gravitational energy of shrinkage 
into molecular motion. 
In a recent paper Stas maintains that electric spectra are to be re- 
garded as distinct from flame spectra; and from researches of his own, 
that the pairs of lines of the sodium spectrum other than D are pro- 
duced only by disruptive electric discharges. As these pairs of lines 
are found reversed in the solar spectrum, he concludes that the sun’s 
radiation is due mainly to electric discharges. But Wolf and Diacon, 
and later, Watts, observed the other pairs of lines of the sodium spec- 
trum when the vapor was raised above the ordinary temperature of 
the Bunsen flame. Recently, Liveing and Dewar saw easily, besides 
D, the citron and green pairs, and sometimes the blue pair and the 
orange pair, when hydrogen charged with sodium vapor was burning 
at different pressures inoxygen. In the case of sodium vapor, there- 
fore, and presumably in all other vapors and gases, it is a matter of 
indifference whether the necessary vibratory motion of the molecules is 
produced by electric discharges or by flames. The presence of lines in 
the solar spectrum which we can only produce electrically, is an indica- 
tion, however, as Stas points out, of the high temperature of the sun. 
We must not forget that the light from the heavenly bodies may con- 
sist of the combined radiations of differentlayers of gas at different tem- 
peratures, and possibly be further complicated to an unknown extent 
by the absorption of cooler portions of gas outside. 
Not less caution is needed if we endeavor to argue from the broaden- 
ing of lines and the coming in of a continuous spectrum as to the rela- 
tive pressure of the gas ia the celestial atmospheres. On the one hand, 
it can not be gainsaid that in the laboratory the widening of the lines 
in a Pliicker’s tube follows upon increasing the density of the residue 
of hydrogen in the tube, when the vibrations are more frequently dis- 
turbed by fresh encounters, and that a broadening of the sodium lines 
in a flame at ordinary pressure is produced by an increase of the quan- 
tity of sodium in the flame; but it is doubtful if pressure, as distin- 
guished from quantity, does produce an increase of the breadth of the 
